The word is an abbreviation of infangenthef, and is also found in English (but only from the mid sixteenth century).
s.
1lawinfangthief, the right of a lord to claim for trial a tenant who has committed an offence within the boundaries of his territory (and to keep any forfeited properties):
(
1321
) Infang et houtfang ne cheit sauntz franchise aver
Ed II xxvi.ii 209
infang (1321)