Without a companion; alone or solitary without another person's company.
From companion (from Old French compagnon, meaning 'one who shares bread') plus the suffix -less (meaning 'without'). The word evolved to describe the state of being without a companion.
Companionless appears in 19th-century literature as a romantic word for loneliness, but it's rarely used today—we say 'alone' instead. Language reveals what matters to us: we had poetic words for emotional states people actually experienced back then!
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