でかした (dekashita) is a somewhat old-fashioned expression that means “good job”, “great work, or “well done”. It contains a very light tone as described below, and is usually used when you praise someone younger than you.
When we hear this word, we picture a man in his 50s or older casually praising his subordinate at work. Therefore, this word is often used in anime or manga to illustrate a stereotypical old man or masculine woman. However, some young people also use this word to sound casual and playful, as in the second example sentence below.
You: I’ve just finished the work that you assinged to me last week!
Your Boss: Well done!
Your little brother: I bought you an ice cream on my way home.
You: Oh really? Great job!
Synonyms: よくやった (you did very well),
Synonyms in English: good job, great work, well done
Related words and phrases: でかしたぞ