1960 V. Anderson Daughters of Divinity vi. 51 And show her where to fill her hotty? 1967 R. Harris All my Enemies xiii. 117 You look rotten, Jenny. At least have a hottie to clutch.
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"A hot water bottle" : Don't leave home without it -- especially if it's winter.
In earlier days, a hot brick wrapped in a flannel (thick cloth) was de rigeur to keep toes toasty!
And the thought occurs there are many meanings for flannel including my Grandmothers phrase when poo-pooing something "That's all flannel (nonsense)."

(This post edited after egregious error pointed out, most sensitively, in a Private Message.)
wow