The word 'stepbrother' is a compound formed within Old English, combining the prefix 'stēop-' with the noun 'brōþor'. The Old English prefix 'stēop-' originally carried connotations of loss or bereavement, specifically referring to orphaned children. This evolved to describe the new, non-biological familial bonds formed when a widowed or divorced parent remarried, with the step-relationship arising from the "loss" of the original parent. Thus, a stepbrother is literally a "brother by remarriage," a male sibling connected not by blood but through the union of one's parent with a new spouse who has children from a prior relationship.

Structure: step (related by remarriage) + brother (male sibling) - step: From Old English 'stēop-', meaning "orphaned" or "bereaved," used as a prefix to denote a relationship created by the remarriage of a parent. Its functional role is a kinship modifier. - brother: From Old English 'brōþor', meaning "male sibling." Its functional role is the core noun denoting the familial relationship.