胡同 is believed to come from Mongolian 'hottog' meaning 'water well' - settlements formed around wells during the Yuan Dynasty. The hutongs of Beijing are a UNESCO-recognized cultural heritage.
grammar
The 儿 suffix (erhua) is the hallmark of Beijing Mandarin. Without it, 胡同 is the standard form used across China.
usage
走街串巷 means to wander through streets and alleys - a common activity for tourists in Beijing's hutong areas.
