The unsolicited explanation of something in the mountains, in an overconfident manner, regarded as condescending, patronizing and often inaccurate.

Plus a total disregard and unwillingness to accept the other persons prior knowledge or unique experience of interrelated fields.
"Are you mountainsplaining me? You may have lived in the mountains longer than me, but that doesn't mean you have to mountainsplain everything to me!"
by Ali Sea March 5, 2021
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