Takoyaki stand, Osaka, 9 p.m. The owner asks how it is.
Option A: "本当に美味しいです。" He nods. Correct. Polite. Transaction complete.
Option B: "ほんまに美味しい！" He stops. He looks up. He shouts something to his wife in the back. You are now getting extra takoyaki and possibly a life story.
ほんま is simply the Kansai form of 本当 (hontou, "really/true"). ほんまに = really. ほんまか？ = seriously? ほんまや = it's true.
The reason it lands so hard has nothing to do with linguistics. Osaka has spent decades being characterised by national TV as the loud one, the funny one, the not-Tokyo one — and Kansai people are proud of it in a slightly defensive way. A foreigner choosing their version instead of the standard one reads as a compliment to the whole city.
Tokyo does not work like this. Speak beautiful standard Japanese in Tokyo and you will get a polite "上手ですね" and the conversation continues at the same distance. Use ほんまに in Osaka and the distance changes immediately.
If you only ever learn one dialect word, this is the one, because it fits anywhere: shops, bars, taxis, konbini, anywhere someone asks how something is.
Foreign learners obsess over politeness levels — keigo, humble forms, the correct suffix. But in Japan, the fastest route to a real conversation is not being more correct, it is being more local. Perfect standard Japanese proves you studied. Slightly wrong Osaka Japanese proves you are here.