The idiom
off and on is idiom # 1412. The
link that David provided above, and the link in this sentence, will take you to the English version of the idiom. You can also go to the idiom dictionary and look up the idiom by its number (1412) using the Advanced Search link.
The reason you can't find the idiom is because it only contains common words, also known as stopwords, that are excluded during searches. Words like
the,
what,
and,
a,
an,
in,
on, and
off are stopwords, and are ignored. I'm not sure if this is something that David can change.