wikipedia -
Sam Hill is an American English slang phrase, a euphemism or minced oath for "the devil" or "hell" personified (as in, "What in the Sam Hill is that?").

oh no! Wikipedia is wrong - CantonDaily Ledger sez. Where did the expression Sam Hill come from?
(Sam Hill was a Michigan surveyor in the 1800s who “allegedly used such foul language that his name became a euphemism for swear words,” according to Wikipedia, so it has to be true. Another source says it's a euphemism for “hell.”)

this is fun! A google search on Sam Hill rings up lots of interesting stuff, a veritable rabbit hole ...