Fenn is usually used in conjuncture with the word 'that', therefore making it 'fenn that'. Its means; give me that, lend that to me, give a bit of that etc.
9 times outta 10 its used to ask for tokes on a zoot a.k.a spliff.
''Boi u got weed? Fenn that!''
''Oi fenn me ur lighter pls''
''I need roach material. Sum1 fenn me that''