Kuromon Market, 7 a.m. A regular steps up to the fish stall before she has even finished deciding what she wants.
"まいど！" the owner says, already reaching for the mackerel she buys every Thursday.
Twenty minutes later she leaves with her bag. "まいど、おおきに！" he calls after her — same word, completely different job, and somehow everyone in earshot understood both instantly.
Maido is short for 毎度 (maido), which just means "every time" or "each occasion". On its own it used to sit inside longer, formal sentences: 毎度ありがとうございます ("thank you, as always") or 毎度お世話になっております ("thank you for your continued support"), the kind of line a shopkeeper or a company rep would say to a client they deal with regularly. At some point in Osaka's long run as Japan's merchant capital, the opening two syllables broke off and started doing the whole sentence's work by themselves.
What is left after the trimming is oddly flexible. Say it when someone walks into a shop and it is "hello, good to see you again". Say it as they leave and it is "thanks, see you next time". Pick up a business phone with it and it replaces もしもし (moshi moshi) entirely — a lot of small businesses answer the phone with まいど, company name, and nothing else, because in a sales call there is no reason to spend a syllable confirming that yes, this is a telephone. Among male friends in Osaka it also moonlights as a flat, casual "hey" — closer to a nod than a greeting.
The common thread across every one of those uses is the word's literal meaning: every time. Unlike a fresh "good morning", which politely pretends this encounter stands on its own, まいど openly states that it does not — that this is one instance in a series, that there was a last time and there will be a next one. It is traditionally a man's word, and it clusters heavily around small, independent trades: fishmongers, butchers, greengrocers, delivery drivers, sales reps — anyone whose whole business depends on the same faces coming back.
That is also why it sounds slightly wrong out of context. Say まいど to a clerk at a convenience store chain in Tokyo, someone you have never met and statistically may never meet again, and the word has nothing to attach itself to. It is not incorrect Japanese, it just has no referent — there is no "every time" yet. The greeting is built for relationships that already have a history, and it simply does not fire correctly without one.
Most greetings are designed to be polite to a stranger — "good morning" works identically whether you have known someone forty years or forty seconds. まいど is built the other way round: it only makes sense once there is a relationship to refer back to. Osaka's merchant culture apparently decided that the warmest thing you could say to someone was not a fresh hello, but proof that you remembered every previous one. That is the whole plot of the city's famous bluntness, running quietly through a single two-syllable greeting.