Hi Haz,
A sensible answer;
Sometimes in climbing, friction plays an important part in one's ability to use a specific hold. Ergo, if it is too warm, some holds will be harder to use than others because the friction between the skin and the rock will be poorer.
A cynical answer and the answer I first wrote before taking pity on your noob status;
This expression would generally refer to the user of said hold complaining that they were not able to hold the hold because they are too shit but not wishing to admit that they were too shit thereby formulating a clever sounding excuse for said weakness. Conditions based excuses for being unable to complete a problem are some of the most commonly used i.e. "It's too warm" or "It's too cold".
There are many other excuses used in bouldering and an experienced user of excuses may be known as a "hustler". Experienced hustlers will very often begin the art of hustling even before the climbing has started e.g.
"I slept really badly last night"
"That injury I picked up the other week has been really bad this morning"
"Work's really taking it out of me at the moment"
It's worth learning the art of hustling early on in your career, particularly if you intend most of your climbing to be done in Britain or with British boulderers...
