The origin of phraseological units in the English language

Phraseological units may come from the following spheres :
1. From engineering and technology
Ex:to let off steam

Ex:to put the plough before the oxen
2. From agro-cultural Ex:to put the plough before the oxen

3. from sea-travelling business
Ex:to be in low waters

4. from trade and commerce
Ex: into bargain

National customs and traditions
Ex: by hook or by crook

from the words of Greek and Latin classics, from the Bible and from myths and legends
Ex: the apple of discord