Dictionary.com
definitions

alongside

[uh-lawng-sahyd, uh-long-]
adverb
  1. along, by, at, or to the side of something: We brought the boat alongside.
preposition
  1. beside; by the side of: The dog ran alongside me all the way.
  2. Informal. alongside of, compared with: Alongside of his brother, he is no student at all.

Origin of alongside

First recorded in 1700–10; along + side1
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for alongside of

alongside

preposition
  1. (often foll by of) along the side of; along besidealongside the quay
adverb
  1. along the side of some specified thingcome alongside
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Word Origin and History for alongside of

alongside

adv.

1707, from along + side (n.). A word formed from a phrase. Originally mostly nautical.

Online Etymology Dictionary, © 2010 Douglas Harper
Idioms and Phrases with alongside of

alongside of

1

Beside, next to, as in Tom's canoe lay alongside of mine. [Late 1700s]

2

Together with, as in Her children played alongside of mine all summer long. [Late 1700s]

3

Compared to, as in My car doesn't look like much alongside of Dad's. [Colloquial; late 1800s]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.