Tally-ho is a greeting commonly used by members of the working class in Great Britain. It is a relatively new word and its use is becoming more and more widespread each year. The internet age has aided this substantial growth, as tally-ho now commonly replaces more formal, older, outdated greetings such as sup alrite
Alternatively, Tally-ho can be used as an exclamation, in situations where the speaker is in great surprise. It is commonly found in the equestrian world, where tally-ho is a variant form of 'lets-go' or 'allons-y'
Adam comes into a chatroom
"Ben: Tally-ho, Adam"
"Charles: Greeting Adam"
"David: yes, a good day to you Adam"
Adam is being ridden by Becky, fiercely and vigorously, and Becky has taken Adam by surprise.