Act I: French Scene 4
disguis'd: Orlando conceals his identity at the wrestling match until directly asked by Duke Frederick.
a fall: one bout, one round of a fight
aquit him well: make a good showing for himself
foyle: (foil), defeat
withall: with everything, with it (also meaning, in addition to, nonetheless, moreover)
intendment: intent, intention, purpose
brooke such disgrace well: take with a good grace
by underhand means: indirectly, without attempting straightforward argument lest it strengthen his determination
stubborn-est: most obstinate and inclined to violence to gain his way; Shakespeare often uses the word 'stubborn' in an earlier sense which implied fierce opposition rather than mere immovable determination.
envious emulator: envious often means 'malicious'; emulator meaning an imitator or envious of another, also a competitor; Oliver accuses Orlando of pretending to be a good man but only imitating a good man to hide his malicious/envious intentions; he is jealous of virtuous people and so pretends to be one, ironically descriptive of Oliver at this point in the play.
naturall: by birth
I had as liefe: I would be just as pleased if, would rather
practise: plot
entrap thee by some treacherous device: Orlando will entrap (trick, ambush or ensnare) Charles by some treacherous device (stratagem, plot, evil plan). And/or entrap (screw) Charles and emasculate him (de- is a prefix for "carry off" and "vice" means "penis").
indirect: underhand, dishonest
goe alone: walk upright without aid; Charles intends to use his strength and skill to insure Orlando will never walk again if he is not killed