This is the 6099th most frequent Japanese word.
エライ
"エライ" means admirable, distinguished, great, or significant; can also mean serious or troublesome depending on context.
In this sentence, 'エライ' is used informally to mean 'very much' or 'extremely' to emphasize the degree of tiredness.
今日はエライ疲れたね。
Today was really exhausting.
Here, 'エライ' is used to describe someone who is respected, esteemed, or has a high status.
先生はとてもエライです。
The teacher is very admirable.
In this context, 'エライ' refers to someone who holds significant authority or is highly esteemed.