Having a passionate nature, or being inclined to quick temper.
Score another for the Bard of Avon. Shakespeare was fond of combining simple words into expressions of poetic imagery (sorry sight, fancy free, primrose path, to list just a few) - he was a consummate poet of course. 'Hot-blooded', or a Shakespeare wrote it 'hot-bloodied', first appears in The Merry Wives of Windsor, 1600:
Falstaff: The Windsor bell hath struck twelve; the minute draws on. Now, the hot-bloodied-Gods assist me!
The Dutch word 'heetbloedig', meaning 'passionate; hot-tempered' is recorded from 1619 (as heetbloedigh). It may be that Shakespeare got the word from the Netherlands but, given the dates and his track record, it is more likely that the expression travelled in the other direction.
See other phrases and sayings from Shakespeare.