Mae fy nghefn yn edrych yn fawr yn...
Breakdown:
* Mae: This is the verb "is" or "are".
* fy: This is the possessive pronoun "my".
* cefn: This means "back", but is often used to refer to the buttocks.
* yn edrych: This means "looks".
* yn fawr: This means "big".
* yn...: This means "in..."
Example:
Mae fy nghefn yn edrych yn fawr yn y dilledyn hwn. (My bum looks big in this dress.)
Note: It's important to remember that while "cefn" can be used to refer to the buttocks, it's a more polite and less direct term than "bum".