(of a person's professional qualifications, ethical standards etc, or of the quality of a work of literature, art etc) of very low quality, base, devoid of merit etc
of the worst (lowest, basest) type (kind, sort).
Это не поэт, а версификатор низкой пробы. He's no poet, he's a rhymster of the worst sort.
Большой русско-английский фразеологический словарь. — М.: ACT-ПРЕСС КНИГА. С.И. Лубенская. 2004.