חִזְק֣וּ וְאִמְצ֔וּ אַל־תִּֽירְא֥וּ וְאַל־תַּעַרְצ֖וּ מִפְּנֵיהֶ֑ם כִּ֣י ׀ יְהוָ֣ה אֱלֹהֶ֗יךָ ה֚וּא הַהֹלֵ֣ךְ עִמָּ֔ךְ לֹ֥א יַרְפְּךָ֖ וְלֹ֥א יַעַזְבֶֽךָּ׃ (פ)
Be strong and of good courage, fear not, nor be affrighted at them; for the LORD thy God, He it is that doth go with thee; He will not fail thee, nor forsake thee.’
Rashi on Deuteronomy
לא ירפך means He will give you no looseness that you should be abandoned by Him (cf. Rashi on Deuteronomy 4:31).
Sforno on Deuteronomy
לא ירפך, for at the hour of battle G’d will lend you additional mental fortitude
Siftei Chakhamim
He will not precipitate, in you, the feebleness [ירפך], etc. Rashi did not explain [the letter chaf] is in place of you [i.e. meaning He will not weaken you] as he normally does, that its meaning is that He will not make you weak before Sichon and Og. It seems that this is because whenever a reason is given it refers to what precedes it, and it is written Do not quake before them [and the reason is] because He will not make you weak. For this reason, “Do not quake before them.” is understandable if the meaning of yarpecha is that He will not give you weakness, to be abandoned from Him. Then it is understandable, for this reason “Do not fear,” because you are not [being] abandoned by Hashem. But if the meaning is He will not make you weak the phrase Do not quake cannot apply, because this is no reason not quake. This is also the explanation of R”em.