kundividi

Decoded: kun plus dividi. Both pieces are already yours - kun at rank 19, dividi at 1,240 - and this is one of the neatest compounds in the language, because the two halves pull in opposite directions and the result is exactly right. Dividi is to divide, to split, to force apart. Put kun on the front and the splitting becomes joint: you divide it with someone, which is sharing. English does the same job with an unrelated word and loses the picture.

Prepositions compound freely, and this is the pattern. Kun- is not an affix, it is the ordinary preposition used as the first root of a compound, and Esperanto lets you do that with most of them. Kunlabori is to collaborate, kunveni to assemble, kunsidi to sit in session, kunporti to bring along, kunsento sympathy. The same freedom gives alveni, elpreni, trairi, superforti. Once you notice it, a large slice of the vocabulary stops needing to be memorised.

Dividi alone is not enough. A learner reaching for "share" often writes dividi la panon, and that means to cut the bread up - the focus is the division, not the company. Kundividi la panon is the sharing. The difference matters most in the figurative uses: kundividi opinion is to hold a view in common with someone, and dividi opinion would be to split one. For the online sense of sharing - posting a link onward - Esperanto normally says dissendi, konigi or simply kunhavigi, and kundividi there is an English calque you can avoid.