boss, n.³ — Green’s Dictionary of Slang

boss n.³

[? boss-eye n., i.e. a ‘squint’ at]

a view, a sight of.

1888		‘’Arry on St. Swithin’ in Punch 4 Aug. 49/1: Couldn’t do a lark round with the ladies or git a fair boss at a race.
1890		Music Hall & Theatre Rev. 5 July 7/2: ‘Is there anything does a man’s eysight good like a boss at a well-built gal?’.

put on a boss (v.)

to affect a squint in order to make oneself look more threatening.

1909