It basically means charge.
Unfortunatly from the 1950's to the present day, this has become a common steriotype in america for what British people say when in fact that has been said more often in america than it ever has in Britain.
Mr. Englishman : Fuck off you Bell End
F-35 Pilot (Viper-2): Chalis-1, Viper-2, Tally Ho!
(Aviation) (carried over from foxhunting) The phrase uttered by a pilot advising that he sees the air traffic that has been called out for him
(Aviation) Ground Control: "United 234, your traffic is a 737 at your ten o'clock descending through your altitude."
United 234: "Roger, Tally-ho!"
Pilot: Tallyho old chap!