From spilve (“cottongrass”) + -ens, with irregular change of e > i.
spilvins m (1st declension)
| singular (vienskaitlis) | plural (daudzskaitlis) | |
|---|---|---|
| nominative (nominatīvs) | spilvins | spilvini |
| accusative (akuzatīvs) | spilvinu | spilvinus |
| genitive (ģenitīvs) | spilvina | spilvinu |
| dative (datīvs) | spilvinam | spilviniem |
| instrumental (instrumentālis) | spilvinu | spilviniem |
| locative (lokatīvs) | spilvinā | spilvinos |
| vocative (vokatīvs) | spilvin | spilvini |