prethee: pray you

picke purse: pickpocket; Orlando may be inconstant but he would not picke (illicitly open) a woman's purse (female pudendum) nor would he steal a horse (whore).

verity: honesty, constancy

concave: hollow; also meaning corrosive or hollow ulcers

Worme-eaten: a worm who has eaten through/ruined a nut; also meaning worme-nut (syphilitic) nut (glans penis).

Tapster: tavern-keeper

confirmer of false reckonings: witnesses to false tavern bills; the Tapster (pimp) also confirms (encourages) false (illicit, adulterous) reckonings (sexual intercourse)

question: conversation

all's brave that youth mounts, and folly guides: everything is great when you're young and foolish; play on mount (meaning copulate) and folly (meaning lust).