I found this interesting from the OUP word blog:
Both these punctuation marks [the ! and the ?] were originally manuscript abbreviations in Latin texts.
To take the exclamation mark first: it derives from a vertical version, written in the margins, of the Latin word Io, meaning ‘exclamation of joy’. The vertical stroke was the ‘I’ above the ‘o’, in which the ‘o’ eventually became a dot.
The question mark, meanwhile, goes back to the word Quaestio meaning ‘questioning, investigation’ and indicating a question. Quaestio was eventually abbreviated to a curly ‘q’ above the ‘o’, while the ‘o’ became a dot.
Why were these marks needed in the first place? Their prime function in Latin manuscripts was to show people the right intonation: a question, a surprise, a shout, etc. If you combine them you get a different effect: ?! delivers an immediate sense of incredulity: a little like smileys or emoticons in our text messages today.
This is an excerpt from What Made the Crocodile Cry? by Susie Dent.
