Relating to or of the nature of depression; characterized by or causing depression.
From depression + '-ary' (adjective suffix). The '-ary' suffix is Latin-derived and creates adjectives meaning 'relating to' or 'of the nature of.'
This is a formal, somewhat archaic cousin of 'depressing'—you're more likely to see it in 19th-century medical texts describing depressionary illnesses than in modern English.
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