A 'notion' is something that has become known to the mind — from Latin 'noscere' (to come to know).
A conception of or belief about something; a vague awareness or understanding; an impulse or inclination.
From Latin "nōtiō" (a becoming acquainted, an examination, an idea, a conception), the noun of action from "nōscere" (to come to know, to learn), earlier "gnōscere," from PIE *ǵneh₃- (to know). This root is among the most prolific in Indo-European, generating Greek "gnṓsis" (knowledge, whence "gnostic" and "diagnosis"), Sanskrit "jñā́na" (knowledge), Old English "cnāwan" (to know, whence "know"), Latin "cognōscere" (to recognise, whence "cognition"), and "ignōrāre" (to not know, whence "ignore"). The Latin noun "nōtiō" was a technical term in Stoic philosophy
In American English, 'notions' (plural) developed a unique meaning: small useful items like buttons, pins, thread, and ribbons sold in a general store. The 'notions counter' was a fixture of nineteenth-century American retail. The connection may be that these items were things one had the notion to buy — small impulse purchases — or simply small clever inventions.