The lyric “Which nobody can...” is completed by “deny” in the American version of “For He's a Jolly Good Fellow.”
The song is a congratulatory standard, commonly sung for birthdays, weddings, retirements, promotions, and victories. Its melody comes from the French song “Malbrough s'en va-t-en guerre,” associated with the Duke of Marlborough and dating back at least to the 18th century.
A common mix-up is treating “deny” as the only ending. British singers traditionally use “and so say all of us,” while “which nobody can deny” is especially associated with the American version. Both variants are established, but the wording in this question specifically points to “deny.”
The song was already familiar in the United States by 1862 and had become a widely recognizable social song by the mid-19th century. Its pronouns can also be changed to suit the person or group being celebrated.