E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A decoy in an auction-room; so called because he buttons or ties the unwary to bargains offered for sale. The button fastens or fixes what else would slip away.
The button of the cap. The tip-top. Thus, in Hamlet, Guildenstern says: On fortunes cap we are not the very button (act ii. sc. 2), i.e. the most highly favoured. The button on the cap was a mark of honour. Thus, in China to the present hour, the first grade of literary honour is the privilege of adding a gold button to the cap, a custom adopted in several collegiate schools of England. This gives the expression quoted a further force. Also, the several grades of mandarins are distinguished by a different coloured button on the top of their cap.
Button (of a foil). The piece of cork fixed to the end of a foil to protect the point and prevent injury in fencing.