Churchy LaFemme: a mud turtle by trade; he enjoys composing songs and poems, often with ridiculous and abrasive lyrics and nonsense rhymes. His name is a play on the French phrase Cherchez la femme, («Look for the woman»). Perhaps the least sensible of the major players, Churchy is superstitious to a fault (for example, panicking when he discovers that Friday the 13th falls on a Wednesday that month). Churchy is usually an active partner in Howland’s outlandish schemes, and prone to (sometimes physical) confrontation with him when they (inevitably) run afoul. Churchy may have once been a buccaneer, because for a time in the early strips he wore a pirate’s hat and was sometimes referred to as «Cap’n LaFemme.» This seems incongruous for the guileless Churchy, however, who is far more likely to play-act with Owl at being a pantomime pirate than the genuine article.

en.wikipedia.org

Собственно, жирным отмечен оборот, который встречается довольно часто, меня он долгое время смущал и я, наконец, погуглил о чем вообще речь идет и наткнулся на совершенно новое для меня значение этого слова, по мимо торговли.

trade [treɪd] сущ. — занятие, ремесло, профессия.

jeweller by trade — ювелир по профессии
They’ve completely ruined the tourist trade for the next few years. — Они полностью разрушили туристический бизнес на последующие несколько лет.
He learnt his trade as a diver in the North Sea. — Он обучился профессии водолаза на Северном море.

Синонимы:
profession, occupation, craft

Плюс к тому, выяснилось вот на этой странице, что это выражение применимо только к работе/профессии связанной именно с физическим трудом, а не с умственным, это вообще очень хорошо вписывается в характер персонажа, особенно если учитывать, что он периодически использует эту фразу в рассказе о себе. Тяжелая работа — быть черепахой, и не поспоришь.

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Anonymous:
I often hear ‘He is a carpenter by trade.’ or ‘He is a blacksmith by trade.’.
Is the idiom ‘by trade’ used only for skilled workers such as carpenter or blacksmith?
Can’t you use it for white-collar workers such as sales person, secretary or researcher?
For example, how about ‘I’m a pharmacist by trade.’?

Yankee:
I’d say ‘by trade’ is usually only used to refer to an occupation involving skilled manual or mechanical work. I’d never say ‘a secretary by trade’ or ‘a researcher by trade’, but I might possibly say ‘a pharmacist by trade’ — probably due to the fact that I’ve heard many pharmacists complain that their job seems to consist primarily of manually measuring or counting out medication.