According to the UCMP (University of California of Paleontology) there is a divide between the popular everyday version of the word theory, and the scientific definition of theory. The popular version, per the UCMP, suggests that the word theory is simply a guess or a supposition while the scientific definition is "a well-substantiated explanation of some aspect of the natural world." There is a big difference then, between the common application of the word theory, and the scientific application of the word theory. In science a theory carries far more weight than the popular usage of the word.
The AHSD (American Heritage Science Dictionary) goes even further to describe the word theory in its definition section of the word hypotheses. A scientific theory, according to the AHSD, consists of hypothesis and laws. A hypothesis, again according to the AHSD, tries to explain a set of facts with the prospect and intention of testing to give credence to, or discredit those hypotheses. Scientific laws, per the AHSD, are hypotheses that have been proven beyond a reasonable doubt which can never be proven to be 100% true. This would lead us to believe that a scientific theory, however substantiated, and being made up of hypotheses and laws which can never be proven 100 % true, also can never be proven true. Theories can be rejected after evidence discredits them, but never proven true with pure absolution because even the laws and hypotheses, of which they are made, can never be proven absolutely. When a scientist, therefore, proposes a theory, it is with far more gravity than the common usage of the word theory.
Considering that a theory can never be proven true might lead some to believe there is credence in using the phrase "evolution is just a theory" in a derogatory manner. In science, however, nothing is ever proven with absolutism because the nature of science is to always improve our understanding, ideally, without the bias of belief. Still, theories are highly tested, unlike the common usage of the word. In fact, the common usage of the word theory has far more in common with the scientific usage of untested hypotheses. I say untested because usually when people use the word theory commonly, they have no intention of testing that theory, they are merely making a loose guess based on minimal information. The point of all this is to say that if something is a theory, in scientific terms, is not derogatory at all to the scientific community at all; instead it is an elevation of an idea to a status higher than when they intended.
In conclusion, when I hear someone say that "evolution is just a theory," I do not get offended at all. If ever evidence discredits evolution as being the means from which all life developed, the unbiased scientist would embrace the idea and do what they can to discover the real nature of the universe. Unwittingly, in scientific terms, the person intending the statement, "evolution is just a theory," as derogatory has in fact paid the idea of evolution a well-deserved compliment as the term theory carries with it the gravity of many well thought out hypothesis and scientific laws.
"hypotheses." The American Heritage® Science Dictionary. 3rd ed. 2005. Print.
UCMP. Misconception: Evolution is just a theory. 2006. Web. 11 September 2012. .
—. Using Appropriate Terminology. 2006. Web. 11 September 2012. .