, seen primarily as とっくに
, is an adverb that can be translated roughly as 'long ago'. This word is an emphasized version of 疾（と）うに, a literary form which is not used much in modern Japanese.とっくに
is a sentence altering adverb, so will appear most often at the beginning of sentences, or before the verb it is describing as having happened a long time ago.
I already ate.
It is already time to sleep. I haven't even started my homework… What should I do?
The due date has long since passed, so it won't matter if I do it or not.
You are late! Everyone has already gone home ages ago!
When compared with structures like すでに
will either indicate something that has happened much further in the past, or may be used simply for exaggeration.
Fun-fact - It is uncommon to see とっくに listed with an exact time, as the stem (old Japanese) word 疾（と）し already means something close to 'far earlier', or 'at a much earlier time'.