It’s fucking hot.
Possibly a rather, er, strong translation?
Boia means “executioner”, and while WordReference mentions un freddo boia as “horrible chill, terrible cold” there is no sign of a caldo version.
It does suggest caldo bestiale.
Seems to be translated as “very warm”.
caldo boia - Translation into English - examples Italian | Reverso Context (There is only one translation for english, but for portuguese and Spanish there are more translations with the meaning of “very warm”.)
caldo boia - Translation into Portuguese - examples Italian | Reverso Context
fa un caldo boia - Traducción al portugués - ejemplos italiano | Reverso Context
caldo boia - Translation into Spanish - examples Italian | Reverso Context
fa un caldo boia - Traducción al español - ejemplos italiano | Reverso Context
Uhm, someone got a bit carried away perhaps! 
So, over at Tatoeba, the thinking behind this is actually recorded in the comments and shows the following sequence of events -
Fa un caldo boia. - Italian example sentence - Tatoeba
There is also a duplicate except for the punctuation (an exclamation mark instead of a full stop), with an English translation of “It’s hot as hell!”
Ah ha, forse, questo è perché Mary sta tralasciando alcuni capi di biancheria intima! Tutto sommato, and now Giorgio is having salsa lessons, really hot stuff;-)
Well… Without seeing the English translation, I came up with the same in my head. It is used in the same way, as an emphatic way to express a feeling.
“Boia” is an intensifier used in many imprecations, and for sure it is milder than the proposed translation, but still, you shouldn’t use in polite speech.
“Bestiale” is used in the same way, and it is less objectionable, similar to “terribile”.
A real eye-opener, thank you.