Green’s Dictionary of Slang

subjected to anal intercourse, usu. by a person with a large penis.

[1859		Hotten Dict. of Modern Sl. etc. 80: ramshackle to shatter as with a battering ram].
1997		Roger’s Profanisaurus in Viz 87 Dec. n.p.: ram shackled euph. To be engaged in the act of botting (qv).