madrastra
/maˈdɾastɾa/ · B2 · noun · f
stepmother
👩👧 madrastra = madre (mother) + -astra, a 'step/substitute' suffix, like 'poetaster'
Spanish madrastra comes from Latin matrastra, built from mater (mother) plus the suffix -astra, which marks a substitute or secondary relation. Spot 'madr-' as 'madre' and you have the meaning: a mother-by-marriage, not by birth.