Why is the noun "brush-off" hyphenated when the verb "to brush off" is not?
Solution 1:
The punctuation mark you're talking about is a hyphen, not a dash, but either would be out of place in phrasal verbs like brush off, where in some meanings, the phrase will divide:
Sarah can't stand Mark, so she brushed him off at the party.
The noun derived from this verb is written with a hyphen: brush-off, but often a word may become so entrenched in the language that the hyphen disappears and the two elements are joined into one compound word. Callback, for instance, is rapidly overtaking the hyphenated form call-back.