"kept in, confined," 1540s, variant of penned, past participle of pen (v.2). Pent-up (also pent up) is from 1580s.
Entries linking to pent
pen v.2
"to confine or enclose in a pen," c. 1200, pennen, from Old English *pennian (only in compounds), from the source of pen (n.2). Related: Penned; penning.