1749, "pertaining to or characterized by sentiment," from sentiment + -al (1). At first without pejorative connotations; meaning "having too much sentiment, apt to be swayed by prejudice" had emerged by 1793 (implied in sentimentalist). Related: Sentimentally.
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.
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Definitions of sentimentalism from WordNet
the excessive expression of tender feelings, nostalgia, or sadness in any form;