長衫，來源於廣東話的發音
為什麼採用廣東話的發音我就不知道了，但是詞典上確實是這么解釋的。可能最早是從廣州流傳到歐美地區的吧，雖然起源於北方，但是那時可能還沒有對應的英文稱呼。
下面是金山詞霸上面的解釋，你仔細看最後的一部分，關於普通話和廣東話的：
cheongsam
[`tFR:N`sAm]
<漢>n.旗袍
cheongsam
cheong.sam
AHD:[chông」s\adie/m「]
D.J.[7t.%806s$8m]
K.K.[7t.%06s$m]
n.（名詞）
A long dress with a high collar and slit skirt, traditionally worn by Chinese women.
旗袍：中國女性傳統上所穿著的高領開衩裙式連身服飾
Chinese (Cantonese) ch'e¿ng shaam [long gown]
Chinese (Cantonese) ch'e¿ng shaam [long gown]
equivalent to Chinese (Mandarin) cháng [long]
equivalent to Chinese (Mandarin) cháng [long]
Chinese (Mandarin) sh³n [gown]
Chinese (Mandarin) sh³n [gown]
cheongsam
cheong.sam
AHD:[chông」s\adie/m「]
D.J.[7t.%806s$8m]
K.K.[7t.%06s$m]
n.
A long dress with a high collar and slit skirt, traditionally worn by Chinese women.
Chinese (Cantonese) ch'e¿ng shaam [long gown]
equivalent to Chinese (Mandarin) cháng [long]
Chinese (Mandarin) sh³n [gown]