To be het up is to be sorely vexed or excited about something and American sources date the expression from the early 20th century. The OED on the other hand weighs in with het as the past participle of the verb to heat and dates it from the late Middle English period, i.e. 1350-1450. Thus, according to the OED, het means heated, which is very close to what we mean when we say someone is het up over something or other. Het up however remains a predominately American expression and on this occasion American sources are preferred to the rather more esoteric findings of the OED.