The verb stem of BOB v.2 or 3, used to denote sudden action.

1

1673.  Marvell, Reh. Transp., II. 253. Turne but over the Leaf and you meet full bob; ‘Reverendissimo in Christo Patri et Domino.’

2

1872.  Baker, Nile Tribut., ii. 32. Bob! and away it went.

3