Green’s Dictionary of Slang

to gain admission to an event or service (e.g. a bus ride) without a ticket and without paying; thus jibber n., one who gets in for free.

1973		Greasy Truckers Live At Dingwall’s Dance Hall [album sleeve notes] n.p.: About 300 humans had been invited that night, but add another 150 or so who had ‘jibbed in’ and you’re nearer the mark. Dingwall’s manager, Colin, and his very hungry bouncers, were hard-pressed that night.... [...] but jibbing was hard, that night, and many were turned away.
1986		D. Goodman interview in Green Days in the Life [tape transcript] I was the king of the jibbers, I could get in anywhere, talk my way in anywhere.
1998		K. Sampson Awaydays 49: You’d hear them arguing in the distance about whether to jib the bus.
2002		Young Magpies Zone 🌐 He jibbed in at the turnstiles, since Jonno had his ticket, and since I cannot see how he could possibly have got in UNDER the turnstiles, I can only assume he stepped OVER them.