EDIT: Also refer to Aminopterin's answer and Travis Hu's answer for more insights.
After some research, I found two reasonable explanations. But, IMHO, the two should be compiled as the following:
老 is a prefix that is added to make 虎 and 鼠 easier to pronounce; besides, it implies that people respect 虎 and fear 鼠.
The two explanations as follow:
##Affix for smooth pronunciation##
Affix for smooth pronunciation
In the Classical Chinese era, texts were concise and some, if not most, of the words were one-character words. Yet, in colloquial context, people prepended/appended affixes to make the words easier to pronounce. These "new" words then became widely accepted and conventional.
Some common affixes and their associated words:
- 子(suffix)
獅子、兔子、鴨子、猴子、鏡子、桌子、椅子、房子、筷子、刀子...
lion, rabbit, duck, monkey, mirror, table, chair, house, chopsticks, knife...
- 老(prefix)
老虎、老鼠、老鷹、老師...
tiger, rat, hawk, teacher...
- 頭(suffix)
甜頭、苦頭、舌頭、骨頭、石頭、木頭...
sweetness, suffering, tongue, bone, stone, wood...
##Affix with connotation##
Affix with connotation
In Chinese, 老
, apart from aged/experienced
, has the implicit, figurative meaning of respected
because people respect elderly so as to comply with the social value. That said, people prepended 老
on the ones they respect. This is why 老虎
and 老師
are named the way they are.
老鼠
, on the other hand, has various interpretations. Some state that it fits to literal meaning of 老
– old
. According to the Compendium of Materia Medica(本草綱目)
, a medical book, rats have the longest life span. Such misunderstanding came from the impression that rats have a high
rate of reproduction and have been around people back then for the longest time.
Some argue that the 老
in 老鼠
implies the cunning characteristics of rats. This interpretation can also be applied to 鷹
. The rest argue that people chose 老
for 老鼠
to imply their fear toward rats and that rats are not easy to deal with.
As to why 獅
is named 獅子
but not 老獅
, the reason is intuitive – to avoid conflicting pronunciation with 老師
.