even at the best of times; already; under normal circumstances
Emphasizes that a situation is already difficult or bad, and an additional factor makes it worse. Often used in patterns like たださえ～のに or たださえ～だから. The glosses 'in addition to' and 'as it is' are less common and can be understood as part of the core meaning.
See also: ただでさえ
I was already busy, and then an unexpected visitor came, so it was tough.
It was already a cold day, so when the wind blew it felt even colder.