Contextual meaning of a few difficult phrases.
‘jealous in honour’ : The young man takes great care of his honour, his reputation as a good man.
‘sudden and quick in quarrel’: ‘quarrel’ is a slightly old-fashioned word for an argument; ‘sudden’ here means unpredictable - so in an argument this young man might suddenly become violent.
‘Seeking the bubble reputation’: a bubble is empty, so by 'seeking the bubble reputation', Shakespeare means a short lived glory.
‘Even in the cannon's mouth’: a 'cannon'
large gun, and its 'mouth' was at the front -so the man seeks his reputation even if it means standing in front of guns.
‘capon lin’d’: ‘to line’ means to fill something at the edges here, the man was fat from eating good chicken.
‘lean and slipper'd pantaloon’ : this phrase describes a thin old man.
‘shrunk shank’: - 'to shrink' means to grow smaller, and a 'shank' is a piece of meat cut from a leg of an animal - so man's legs have grown narrower with age.
‘treble’: a treble is the higher part of a piece of music - so Shakespeare is referring to a boy's high-pitched voice.
‘oblivion’: if someone is 'oblivious' he/ she doesn't know what is happening around him/her, and if he/she lives in 'oblivion' he/she is completely forgotten by other people.