From Latin 'sōlus' (alone, only), via Old French — the root behind solo, solitary, desolate, and even sullen.
One and only; single.
From Old French 'soul' meaning 'alone, single,' from Latin 'sōlus' meaning 'alone, only, sole.' The same root gives 'solitary,' 'solo,' 'desolate,' and 'sullen' (originally 'alone'). Key roots: sōlus (Latin: "alone, only").