！。
Sh-shoot! My tongue just slipped.
Literal
Sh-shoot! Accidentally mouth [subject-が] slipped.
A vivid slice of spoken Japanese. し、しまった is a stammered/breathed form of しまった ('damn!' 'I blew it!'), with the comma marking a stumble — the character is reacting in real time. ぁ at the end elongates the vowel for emphasis. 口がすべる is a set idiom literally 'the mouth slips,' meaning 'to let something slip (that one shouldn't have said),' 'to blurt out inadvertently.' つい is an adverb meaning 'unintentionally,' 'without thinking,' 'accidentally' — it marks the action as done without deliberation. Perfect fit with 口がすべる. Together: 'Oh shoot, I accidentally let it slip.'