1
general::
Phrase(s): first off
first; the first thing. (Almost the same as first of all.) • He ordered soup first off. • First off, we’ll find a place to live.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
From the start, immediately. For example, He said to wash the car first off, or Why wasn't she told first off? [Colloquial; late 1800s] Also see under FIRST AND FOREMOST; FIRST THING.
American Heritage Idioms