The story
From splain, itself from explain, following the pattern of mansplain.
Etymology from Wiktionary, CC BY-SA 4.0.
Today it means: A suffix combined with a descriptive adjective or noun to create a verb meaning someone who fits that description condescendingly explaining something to someone who does not fit that description (especially, something the listener has more experience of). Full definition →
From splain, itself from explain, following the pattern of mansplain.
Etymology from Wiktionary, CC BY-SA 4.0.