tell them that you are ticklish and that you hate people tickling you
trust me i get it all the time
Next time you are alone with them, put a movie on and sit at the other end of the couch to him/her. Put your feet in their lap and ask for a foot rub. React in a ticklish way every so often, but do not remove your feet, just chuckle or twitch a bit.
If s/he apologises immediately say "That's OK. I actually really liked that."
Job done. And you haven't had to bring it up "out of the blue".
Chat with our AI personalities
She's always tickling me.. I don't like it but I don't have a choice she likes to see me laugh.. a lot of times she's calling to my little brother an little sister to hold me up and she's starting to tickle my belly and armpits..