The verb stem of BOB v.2 or 3, used to denote sudden action.
1673. Marvell, Reh. Transp., II. 253. Turne but over the Leaf and you meet full bob; Reverendissimo in Christo Patri et Domino.
1872. Baker, Nile Tribut., ii. 32. Bob! and away it went.