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a combination of the words sincere, and serious. used to convince someone that you mean what you say.
Johnny: You look beautiful.
Susie: oh, you don't mean that!
Johnny: I'm being sinserious.
by bobillard richardsonly July 11, 2010
the words sincere and serious together to indicate something such as a sentiment which is seriously sincere.
I'm Sinserious about this; I'm utterly convinced he did it.
by George D T June 08, 2007