hold down means "keep". That is the primary sense; the word has 2 in total, listed in full on this page.
Is hold down a common word?
hold down is banded "Uncommon" — turns up in careful writing rather than conversation. The band comes from how often the word appears in a large corpus of everyday English, not from an editor's impression.
Definitions, synonyms and semantic relations for hold down come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. .