Also found in: Dictionary.
To achieve a foamy, sudsy, or frothy consistency. The soap started to foam up once I mixed it with water.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
[for something, such as soap or milk] to make foam or lather. Milk will foam up when it is boiled. The boiling soup foamed up and slopped over the pot.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
To become more foamy: Shaving cream foams up when you spray it from the can.
The American Heritage® Dictionary of Phrasal Verbs. Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.