According to Merriam-Webster, a "meal ticket" is something that serves as the ultimate source of one's income. Example:
• "The Mavs got surprisingly little from their meal ticket, Doncic, in the first half as he was hit with his third foul and a technical late in the second quarter after scoring just seven points." -- Tim Cowlishaw, Dallas News, 30 Jan. 2021
The Urban Dictionary puts it another way:
A "meal ticket" is a person, possibly a boyfriend, girlfriend, husband, wife, friend, et cetera, who is kept around and close in order to be used monetarily. Example:
• "Though they had dated for years, John was nothing more than a meal ticket to Betsy. He was just a dumb, naïve fool that was there to be used, nothing more. Stupid fool believed someone actually loved him."
In the latter definition, the term is synonymous with "Sugar Daddy".
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