长衫，来源于广东话的发音
为什么采用广东话的发音我就不知道了，但是词典上确实是这么解释的。可能最早是从广州流传到欧美地区的吧，虽然起源于北方，但是那时可能还没有对应的英文称呼。
下面是金山词霸上面的解释，你仔细看最后的一部分，关于普通话和广东话的：
cheongsam
[`tFR:N`sAm]
<汉>n.旗袍
cheongsam
cheong.sam
AHD:[chông”s\adie/m“]
D.J.[7t.%806s$8m]
K.K.[7t.%06s$m]
n.（名词）
A long dress with a high collar and slit skirt, traditionally worn by Chinese women.
旗袍：中国女性传统上所穿着的高领开衩裙式连身服饰
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]