To be over-protective or smoothering of another individual. A mumsey person will not stop to think how mumsey they are - indeed, they will not see how embarrassing or annoying it can be.
"Make sure you've got everything. I've put enough sandwiches in your suitcase to feed a small army and I've squeezed in two thermos flasks of coffee. Don't forget to call us when you ge back! Have you got everything..." - typical mumsey reaction when I return to London after visiting the parents in Norfolk.