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