I'm familiar with the idiom "burning the candle at both ends" to mean "to have expended oneself, in particular by staying up very very late". With this idiom I usually think of someone who has either been up all night finishing some important work, or conversely enjoying a social life that keeps them awake until the early hours. However, the other day my Gran used the expression to mean being unfaithful in a relationship, as in:
He's been burning the candle at both ends you know? Oh yes, and if his wife finds out there'll be hell to pay!
I thought it quite a funny way to describe someone cheating, but mainly wrote it off as one of those odd things my Gran says. However, Googling the idiom and the word "unfaithful" I actually found a connection. I found a paper about extramarital sex and marriage disruption with the idiom in its title, as well what looks like an advice blog which warns "Burning the candle at both ends is dangerous." with reference to the act of cheating.
I dug a bit further into the origin and usage of the phrase, but I couldn't find anything which gives a definition including infidelity. The original definition seems to have been more about wasting money, candles being expensive and burning them at both ends being a way to use them faster and waste them. Over time this then morphed into the current well-known meaning, that by "burning the candle at both ends" you were rapidly spending yourself rather than rapidly spending money, and consequently you're now exhausted.
Does anyone know when the idiom "burning the candle at both ends" came to include a meaning of being unfaithful in a relationship? Or is this a natural expansion of the primary meaning of the idiom and isn't a special case?
I have found further uses of the phrase specifically relating to having and affair/cheating here, here, and here. I admit it's not a very common phrase for this sort of behaviour, but it does seem to be in use.