The words knickers, nickers sound the same but have different meanings and spellings. Why do knickers, nickers sound the same even though they are completely different words?
The answer is simple: knickers, nickers are homophones of the English language.
Long bloomers formerly worn as underwear by women and girls.
Chiefly British Panties.
Full breeches gathered and banded just below the knee.
Third-person singular simple present indicative form of nicker.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").