Translation:This path goes northeast.
Onyomi refers to kanji readings of Chinese origin.
Japanese kanji have different pronunciations depending on the words that they are used in. The correct way to say the kanji is called a "reading". Most kanji have at least two readings and some have many. A particular kanji character might appear in many different words. Some words share the same readings, but often times the kanji character will be read one way in some words and a different way in other words. Most kanji will have at least one onyomi reading (chinese origin) and at least one kunyomi (native Japanese origin) reading. Which one is more common varies depending on the kanji. Early Japanese borrowed many Chinese words due to cultural exchange with China. These loanwords became incorporated into the language, along side words from native Japanese. When adapting Chinese characters to spoken Japanese, loan words and new words that incorporated Chinese loanwords would sometimes be written with the same kanji characters as related native Japanese words since the Japanese words did not have their own characters. Kanji characters have meanings associated with them, so a suitable character could be chosen to write the Japanese word based on a similar meaning, even if the word was pronounced differently from the Chinese character. This is why one kanji character can have many readings.
Not weird at all. And the answer might help other people when they run into similar issues.
The title of the manga is 北斗の拳 (ほくとのけん) . Notice the kanji.
北東 means "north east" and 北斗 is the Japanese name for a constelation of stars in the northern part of the sky, called the big Dipper or the Plow.
Also, it is worth pointing out that these words sound very similar, but not identical.
北斗 「ほくと」 vs 北東 「ほくとう」
There is a long お sound in 北東 (northeast).