Someone not ready for the real world (the big sea -- they're restricted to the mud, not even ready for wading in the shallows). Used by some in the navy as a disparaging term for marines and coast guard. Used also in amateur radio for people whose broadcast is weak (and therefore come through muddy with too much noise and not enough signal).
Wait here, mud duck, and let me show you how it's done.
NO! not the duck you eat at the table on Christmas. How dare you! It is the magic rubber duck that brings duck related messages and gifts to you, but only if you believe in her (the christmas duck is a girl.) She doesn't only bring gifts on the 25th like Santa. She comes whenever she is called about to bring ducky cheer!
The Christmas Duck will come and leave notes and gifts for a certain duck fan in my dorm all throughout the week.