Green’s Dictionary of Slang

random, fortuitous, also as n. luck, chance.

1825		Pierce Egan’s Life in London 4 Sept. 253/2: [H]e put in, by ‘hab-grab,’ a tremendous blow on the ould one’s gullet.
1874		Eng. Mechanic 28 Aug. 619/1: His information is purely of the hab-grab order, picked up by reading on the subject [...] but experimenting not at all.