Comparative form of gladsome; more happy, cheerful, or joyful.
From gladsome plus -er, the comparative suffix. Though gladsome is archaic, this comparative form can still theoretically be constructed following English grammar rules.
You probably wouldn't naturally say 'gladsomer' in conversation—most people would say 'more gladsome' or just skip the archaic word entirely and use 'happier.' But this form shows how English grammar rules work systematically: any adjective can theoretically take -er.
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