An adverb used to emphasize that a situation is already (often bad or difficult), often meaning 'even as it is' or 'to begin with'.
ただでさえ忙しいのに、さらに仕事が増えた。
I was already busy, and on top of that my workload increased.
ただでさえ難しい問題を簡単には解けない。
You can't easily solve a problem that is already difficult to begin with.
ただでさえ寒いのに窓を開けないでください。
It's cold as it is, so please don't open the window.
ただでさえ人手が足りないので、休まれると困る。
We're already short-staffed, so it's a problem if someone takes time off.