"sexual relations between women," 1890 (as something "found in French novels"), from the name of Sappho; see Sapphic + -ism. Sapphist for "female homosexual" is by 1923.
Entries linking to sapphism
Sapphic adj.
c. 1500, "of or pertaining to Sappho or her poems," especially in reference to her characteristic meter, from French saphique, from Latin Sapphicus, from Greek Sapphikos "of Sappho," in reference to Sapphō, Greek lyric poetess of the isle of Lesbos who flourished c. 600 B.C.E. and was famed for the passion and loveliness of her verse, which survives mostly in fragments. The sense of "pertaining to sexual relations between women" is from 1890s (also see Sapphism, and compare lesbian).
-ism
word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.