Things are fine. A casual response to questions like "How are you?" or "How've you been?" A: "Hey, Pat, how are you?" B: "Ah, can't complain!"
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
(I) can't complain. and (I have) nothing to complain about.
Inf. a response to a greeting inquiry asking how one is or how things are going for one. Sue: How are things going? Mary: I can't complain. Mary: Hi, Fred! How are you doing? Fred: Nothing to complain about.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
Used as a response meaning fairly good or well, to questions such as "How are you?" or "How is business?" For example, How've you been?-Can't complain. This term means that nothing serious is wrong. [Mid-1800s]
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Pretty good, in response to “How are things going?” This very modern-sounding phrase, which means one has nothing genuine to complain about (or at least will not admit it), comes from mid-nineteenth-century Britain. Eric Partridge cites an early example, R. S. Surtees’s Hawbuck Grange (1847), in which one character observes that time is passing lightly over another, who replies, “Middling—can’t complain.” Today it is a frequent response to inquiries about a business. See also fair to middling.
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer