Loud high pitched term of appreciation, ranging from the humble thankyaaah to the truly excruciating thankyaaaaaaaaaaaaaaaaaaaah, first attributes to Patrick Trueman (if you don't know, consider yourself lucky), although this is a tenuous link at best.
(me): Here, have an earwig....
(you): Thankyaaaaaaaaaaaaaaaaaaaaaaaaaah, cough, splutter
(me): here, have an earwig....
(you): Thankyaaaaaaaaaaaaaaaaaaaaaah, cough, splutter