1.
If you ~ part of something, you do not use or give away all of it, so that you still have some to use at a later time.
Roughly chop the vegetables, and ~ a little to chop finely and serve as a garnish.
= set aside, hold back
PHRASAL VERB: V P n (not pron), also V n P
2.
If you keep some information back, you do not tell all that you know about something.
Neither of them is telling the whole truth. Invariably, they keep something back.
PHRASAL VERB: V n P, also V P n (not pron)