Where does “ukłaść” come from?

ukłaść (Polish) comes from Polish u-, from Old Polish u-, from Proto-Slavic u- — away.

Ancestry of “ukłaść”, step by step

Step	Language	Word	Meaning
1	Polish	u-	Appears in front of some verbs meaning: to make...
2	Old Polish	u-	perfective prefix
3	Proto-Slavic	u-	away