After mention was made of this on the AA forum at FlyerTalk:
https://www.aa.com/i18n/customerService/customerCommitment/conditionsOfCarriage.jsp#acceptanceofpassengers American may refuse to transport you, or may remove you from your flight at any point, for one or several reasons, including but not limited to:
[...]
11. Have an offensive odor not caused by a disability or illness.
Naturally, that got me curious.
http://www.delta.com/content/dam/delta-www/pdfs/legal/contract_of_carriage_dom.pdf Delta may refuse to transport or may remove passengers from
its aircraft in any of the following situations:
[...]
6) When the passenger has a malodorous condition
https://www.united.com/web/en-US/content/contract-of-carriage.aspx UA shall have the right to refuse to transport or shall have the right to remove from the aircraft at any point, any Passenger for the following reasons
[...]
H. 16. Passengers who have or cause a malodorous condition (other than individuals qualifying as disabled)
https://www.southwest.com/assets/pdfs/corporate-commitments/contract-of-carriage.pdf Carrier may refuse to transport, or remove from the aircraft at
any point, any Passenger in any of the circumstances listed below as may be
necessary for the comfort or safety of such Passenger or other Passengers and
crew members:
[...]
(vi) Persons who have an offensive odor, unless caused by a disability
You get the idea. I stopped looking there.
Most of them also have a clause about barefoot passengers -- Southwest excuses 5-year-olds and younger. All except Delta also say something about clothing (see also:
here and
here)