The prefix 'En

The prefix 'en' means 'into or in', which gives rise to several derivative words, but the curious thing is that if the prefix is attached to a word that starts with b, m and p, the consonant n in this prefix is replaced by the consonant 'm'!
Some examples are given below:

• enslave - make into slaves
• endanger - put into danger
• enable - make somebody able to do a task
• encourage - give courage to a person
• enfeeble - make a person weak
• enrol/enroll - put someone on the rolls 
• encircle - to surround
• environs- surroundings
• embrace - give a hug
• empower - give some the power 
• embitter - make someone's nature bitter
• employer - person who puts another in a job

and so on!

Knuckle Down

Believe it or not, this phrase has its origin in that childhood game of marbles

This is how you place your hand when you were about to shoot a marble at another one. The idiom means getting on to the job and doing it.
You can see the knuckles touching the ground, which is where the phrase came from.