If thou faint - If thou give way to discouragement and despair in the day of adversity - time of trial or temptation.
Thy strength is small - צר כחכה tsar cochachah, thy strength is contracted. So the old MS. Bible excellently: Gif sliden thou dispeire, in the dai of anguyfs, schal be made litil thy strengthe. In times of trial we should endeavor to be doubly courageous; when a man loses his courage, his strength avails him nothing.

[If] thou (b) faintest in the day of adversity, thy strength [is] small.
(b) Man has no trial of his strength till he is in trouble.

If thou faint in the day of adversity,.... When under bodily afflictions, stripping providences, reduced to great straits and wants; or under the violent persecutions of men, which is sometimes the case of the people of God; whose times are in his hands, times of adversity, as well as prosperity; and which are appointed by him, when they shall come, and how long they shall last; which is but for a short time, it is but a "day", and yet they are apt to "faint" under them, through the number and continuance of their afflictions; and especially when they apprehend them to be in wrath; when they have a sense of their sins at such a time, and no view of pardon; when they are under the hidings of God's face, their prayers do not seem to be heard, and salvation and deliverance do not come so soon as they expected; which, notwithstanding, shows the truth of what is next observed;
thy strength is small; such who are truly gracious are not indeed at such times wholly without strength; they are in some measure helped to bear up; but yet their sinkings and faintings show that they have but little strength: they have some faith that does not entirely fail, Christ praying for it; yet they are but of little faith; they have but a small degree of Christian fortitude and courage; there is a want of manliness in them; they act the part of children and babes in Christ; they do not quit themselves like men, and much less endure hardness, as good soldiers of Christ, as they should; they are, Ephraim like, without a heart, a courageous one, Hosea 7:1. Some think the words have reference to what goes before, and the sense to be this, "if thou art remiss" (g); that is, if thou art careless and negligent in time of health and prosperity, in getting wisdom, as thinking it too high for thee, Proverbs 24:7; "in the day of adversity thy strength will be small"; thou wilt not have that to support thee which otherwise thou wouldest have had. Aben Ezra connects the sense with the following, "if thou art remiss", in helping and delivering thy friend in affliction, Proverbs 24:11; "in the day of adversity", or "of straitness, thy strength shall be strait"; thou shalt be left in thy distress and difficulties, and have none to help thee.
(g) "si remiseris", Tigurine version; "remissus fuisti", Pagninus, Montanus, Mercerus, Gejerus; "si remisse te geras", Junius & Tremellius, Piscator; so Michaelis.

Under troubles we are apt to despair of relief. But be of good courage, and God shall strengthen thy heart.

Literally, "If thou fail in the day of straits (adversity), strait (or, small) is thy strength," which is then truly tested.

The last of these four distichs stands without visible connection:
Hast thou shown thyself slack in the day of adversity,
Then is thy strength small.
The perf. 10a is the hypothetic, vid., at Proverbs 22:29. If a man shows himself remiss (Proverbs 18:9), i.e., changeable, timorous, incapable of resisting in times of difficulty, then shall he draw therefrom the conclusion which is expressed in 10b. Rightly Luther, with intentional generalization, "he is not strong who is not firm in need." But the address makes the proverb an earnest admonition, which speaks to him who shows himself weak the judgment which he has to pronounce on himself. And the paronomasia צרה and צר may be rendered, where possible, "if thy strength becomes, as it were, pressed together and bowed down by the difficulty just when it ought to show itself (viz., להרחיב לך), then it is limited, thou art a weakling." Thus Fleischer accordingly, translating: si segnis fueris die angustiae, angustae sunt vires tuae. Hitzig, on the contrary, corrects after Job 7:11, רוּחך "Klemm (klamm) ist dein Mut" [= strait is thy courage]. And why? Of כסה [strength], he remarks, one can say כשׁל [it is weak] (Psalm 31:11), but scarcely צר [strait, straitened]; for force is exact, and only the region of its energy may be wide or narrow. To this we answer, that certainly of strength in itself we cannot use the word כסה drow eht esu t in the sense here required; the confinement (limitation) may rather be, as with a stream, Isaiah 59:19, the increasing (heightening) of its intensity. But if the strength is in itself anything definite, then on the other hand its expression is something linear, and the force in view of its expression is that which is here called צר, i.e., not extending widely, not expanding, not inaccessible. צר is all to which narrow limits are applied. A little strength is limited, because it is little also in its expression.