IPA: /əˈlɔːŋsaɪd/
KK: /əˈlɔŋˌsaɪd/
Next to or in a position parallel to something else; close to the side of something.
The car drove alongside the bus during the traffic jam.
Next to or beside something; in a position that is next to another person or thing.
The dog walked alongside its owner during the evening stroll.
Alongside is formed from "a-" (meaning on or at) and "longside" (from Old English "langside", meaning the side of something). The word describes being on or at the side of something, indicating proximity or companionship.
Think of being 'on' ('a-') the 'side' ('longside') of something — that's why alongside means being next to or with.