What is the meaning of "his uncle by marriage in the following sentence:
- Tom was adopted by Sam, his uncle by marriage.
Your uncle can be either:
An "uncle by marriage" is an uncle in the second sense.
Whilst it's conceivable that someone might describe their spouse's uncle as their uncle by marriage, this is unlikely to be the answer you're looking for, because normally only children get "adopted", and you referred to someone being adopted by his "uncle by marriage".