You'd think the English would have a better grasp of their language, eh?
Since language is a constantly evolving construct, it could be argued that the meaning of "ingest" is defined by common usage. Why? Well, students of Laccanian psychoanalysis might argue that this is because the link between signifier and signified is fluid and subject to the influence of the imaginary, the real and the symbolic. This is in many ways an evolution of the theories of the structuralist Saussure, who was one of the first to attempt to define meaning as the relationship between signifier and signified, and his work was of course built upon by the later deconstructionists and post-structuralists.
Perhaps, Noel, if you had a better grasp of your language, you would realise this, eh?
For the rest of you: Dictionary definitions are meaningless because they are outdated as soon as they are written. However, prestige dictionaries such as the Oxford English Dictionary are useful because they chart the historic usage and evolution of the words they attempt to define. "Ingest" has a scientific meaning, but, arguably, this is not the only meaning of the word - alternate (potentially conflicting) meanings can be just as valid if they represent common usage.