Hot tooth is:
A tooth that is difficult to anesthetize is known as a “hot tooth”. 
This is most commonly encountered in a mandibular first molar tooth wherein after the anesthetic block, the patient may describe profound numbness of the ipsilateral lip and tongue but still may experience acute pain during the access opening procedure.

Ref: Grossman’s endodontics, 13th edition, pg no:149
Tooth in line of fracture
Difficult to anesthetise
Vertical line fracture
Very sensitive to heat test