Son-in-law character

It is a rare written form for a son-in-law or groom.

Japanese uses 聟 for むこ; Mandarin uses it for the corresponding family relationship. Its Mandarin pronunciation is roughly “shü” with a falling tone, written xù in pinyin, a system for spelling Mandarin sounds.

An early Chinese legal manuscript from the Han dynasty, an imperial period, is dated 52 BCE: 甘露二年丞相御史律令. Other attestations are the Wuwei manuscripts and the 唐公房碑, an inscribed stone monument. In Heian Japan, むこ could mean a husband welcomed into his wife’s household; in medieval warrior society, it increasingly meant a husband brought in as heir when a family had no son. Modern Japanese ordinarily writes 婿.

Is 聟 different from 婿?

No; they write the same Japanese family word.

婿 is the ordinary modern Japanese spelling. 壻 is an older form in the classical lexicographic tradition, while 聟 is recorded as a nonstandard variant. The forms do not mark different family roles.

Use 聟 when reproducing an older source, inscription, or preserved name. For ordinary Japanese writing, 婿 is clearer.

What does 聟 mean when someone sends it?

Read it as a rare spelling of a family relationship.

With no sentence around it, the safest reading is the kinship meaning. If you cannot tell which relationship is meant, ask about the sentence.