From Longman Dictionary of Contemporary Englishfeel somebody ↔ up phrasal verb informalSYto touch someone sexually, without their permission → feel→ See Verb table
Examples from the Corpus
feel up• In addition, Lacan feels that taking up a position with respect to meaning structures is inextricably gender-linked.• The process of recording how you feel sets up an internal dialogue that can offer new insights into old difficulties.• I feel some one come up behind me.• The dog was almost to the rail fence at the edge of the grounds when I felt somebody slip up behind me.• When you have a negative perception about some one, that feeling ends up creating a negative relation-ship between the two of you.• I lie in the meadow, or I kiss your child I draw on feeling banked up for that time.• Several times she felt almost caught up with the constant demands for her attention. 