This joke plays on the word "halálom", which means "my death". "Halálom" is by chance made up of the words "hal" - "fish" and "álom" which is "dream". So "Fishdream" is the same as "My death". The joke is: "Which animal is it scary to dream of? A fish. Why? It's about my death / fishdream."
How does a retired man walk around? - By fumbling around. ("fumbling" is "botorkál" in Hungarian, which starts with "bot", meaning "stick". So "sétabotorkál" is a combination of "walking stick" and "fumbling around").
Otolaryngologist (An ear and throat specialist is "gégészet" in Hungarian, while the sound geese make is "gá gá" in Hungarian, so "gégészet" becomes "gágászat")