Without a halter; not wearing a halter; uncontrolled or unrestricted.
From halter with the negating suffix -less (from Old English 'læs' meaning lacking), following productive English patterns for creating negative adjectives.
The -less suffix is the opposite of -ful, but it's much older in English. 'Artless' (without art/skill) and 'homeless' show how it creates vivid absence rather than just negation.
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