An exclamation praising someone's achievement, similar to 'well done!' or 'good job!'. It is the past tense of the verb 出来す (でかす), meaning 'to accomplish' or 'to pull off'.
See also: 出来す (でかす)
でかした！よくやったね。
Well done! You did a great job.
I heard you got a perfect score on the test? Good job!
Rare kanji spelling; the kana form is strongly preferred.
Past tense of the verb 出来す (でかす), meaning 'to accomplish' or 'to manage to do'. The exclamation でかした is a fixed expression praising a completed action.