Philly and Filly are easily confused words. Ordinarily, they would be homophones, but one of these words is slang.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Philly (pronounced “fill-ee”) is slang for a proper noun. Philly is short for Philadelphia, a very large city in eastern Pennsylvania. A Philly cheesesteak is a hot sandwich served on Italian bread, stuffed with melted cheese and thinly sliced grilled beef. The cheesesteak hails from Philadelphia.
Filly (pronounced “fill-ee”) is a noun. It means a young female horse, typically under four years old. Once a horse is mature and considered ready for breeding, she is called a mare.
Filly is also a slang word for a lively, young woman, but this usage sounds dated in the US. It’s not used to someone’s face.
The following story uses both words correctly:
Phillida was walking down the streets of Philadelphia, munching on a Philly cheesesteak. Behind her, she heard someone whistle, then shout, “Wow. What a filly. Amazing legs on that one.”
She felt embarrassed, then mad. Then she realized that voice sounded familiar. She turned around. It was Felipe, an old friend from her days as an exchange student in Argentina. Both strangers in a strange land, they had connected immediately. He always said things to irritate her.