1) An iron with a flat face for smoothing linen, etc.
It appears to be earlier than the term flat-iron, which has not been noted before 1810. Examples of ‘smoothing-iron’ are on record elsewhere from 1627, and Yorkshire references date from roughly the same period: 1613 one toasting iron, one smoothing iron, Ripley
1628 one smothinge yron, Pudsey
1637 two smoothing irons, Knaresborough. A word with much the same meaning was ‘smoothing-box’ which is on record from before 1700: it did not displace ‘smoothing-iron’ which remained in everyday use until more modern irons were invented. It survives as the business name of a Staffordshire ironing service.