Sidney sits next to 3 phonetic neighbors — Sydney, Cindy, and Sadie are the closest matches by sound. Most share Sidney's English-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"wide island"
"defender of mankind"
"follower of Christ"
"shadow, shade (if derived from Hebrew Zillah)"
"priest's meadow"
"light, splendor"
"heavenly mist"
"hardest substance, precious gemstone"
"wave, sea foam"
"bright, shining light"
"God is my light (Elizabeth origin)"
"blind, Cecilias clan"
"pleasant, agreeable"
"white, blessed + elm tree (hybrid)"
"vast, empty landscape"
"sea (via Kai)"
"pure"
"invented Valyrian fantasy name"
"red clearing"
"life, living"
Similar sound: These names share phonetic patterns with Sidney — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Old English origins, giving them the same cultural heritage as Sidney.
Rhythm: Sidney is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.