Referring to, or relating to a system in a body that is made up of all the cells able to ingest bacteria, collodial particles and other such stuff excepting the leucocytes.
reticulum + endothelial = reticuloendothelial
and also a really long word used to impress those who don't know what it means. Now that you know, you too can impress people at parties. (Just wait for them to back away, smiling and not breaking eye contact.)