Akademik

грех один
ГРЕХ (СМЕХ) ОДИН (, а не...) coll, disapprov
[NP; these forms only; fixed WO]
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one is completely incompetent, unqualified, ill-suited (for the position, role etc specified in the preceding context), sth. is of very poor quality, useless, does not properly fulfill its role or function:
- one (sth.) is a poor excuse for a [NP];
- there's no way you can call one (sth.) a [NP];
- [in limited contexts] one (sth.) is a joke (, not a [NP]).
     ♦ Какой из него начальник! Грех один, а не начальник. Hima boss? He's a joke, not a boss.

Большой русско-английский фразеологический словарь. — М.: ACT-ПРЕСС КНИГА. . 2004.