yimkin: nonsense; I don't believe it. (Of Arabic origin)
Source: LS2

Yakka: work, task
Source: SL

Yard dog: a customs officer; also, an intruder
Source: LS1

Yer wanna: you ought to
Source: LS1

Yew wid de ead! Eh! Yew wid de ead!: Waiter!
Source: LS1

Yews: you(plural)
Source: LS1

ying-tong: Oriental music; also, a disjointed, erratic, and presumably drunken dance
Source: LS2

yip(obsolete): to talk, to sing to the police
Source: A

Yocker, to: spit. Ed yockered on me: he spat at me.
Source: LS1

yodel(obsolete): to sexually abuse
Source: A

Yonks: ages.  "I avent heard that saying in yonks."
Source: SL

yooks and twerns(obsolete): swivels (evidently attractive to thieves)
Source: A

yowler: a cat. A jigger-yowler is an alley cat
Source: LS2

yunuck: frigid, not interested in sex. Spiegl suggests that this is a mispronunciation of "eunuch."
Source: LS2