The wrong side of the blanket. A love-child is said to come of the wrong side of the blanket.
“He grew up to be a fine waule fallow, like mony ane that comes o'
the wrang side o' the blanket.” —
“Treated with a wet blanket,” discouraged. “A wet blanket influence,” etc. A wet blanket is used to smother fire, or to prevent one escaping from a fire from being burnt.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894