Used for judgments, decisions, or conclusions made too quickly without sufficient thought or evidence. Often appears in the phrase 早計に過ぎる (to be too hasty).
彼が犯人だと決めつけるのは早計だ。
It's premature to conclude that he is the culprit.
まだ結果が出ていないのに、成功したと思うのは早計に過ぎる。
It's too hasty to think we've succeeded when the results aren't out yet.
早計 specifically describes a hasty judgment or conclusion, while 軽はずみ covers any thoughtless action or remark, not just conclusions.
Compound of 早 (early, fast) and 計 (plan, measure). The exact historical derivation is uncertain, but the word has been used since at least the Edo period to mean a hasty plan or premature judgment.