Restrained from Old French restreign-, stem of restreindre, from Latin restringere, from re- ‘back’ + stringere "to tie, pull tight". 

A restrained person can be defined in two main ways: as someone who is calm, controlled, avoiding excessive emotion or extravagance, or, in a legal context, as someone who is physically held, restricted, or prohibited from acting by a legal authority or order.  The meaning depends on whether "restrained" describes their demeanor or their physical and legal state. 

According to the Oxford English Dictionary (OED), the adjective restrained was first used in English in the late 1500s,  with the earliest recorded evidence dating to 1578 in a translation by H. Wotton.  Restrained is formed within English by derivation from the verb restrain plus the suffix -ed.  According to the Oxford English Dictionary (OED), the verb "restrain" was first used in English during the 14th century.  The OED's earliest evidence for the word's use in this period is approximately from the time of John Trevisa, a translator who lived before 1398. 

Examples of "restrained" used in a sentence: 
"Finally, he spoke up once more, his voice broken with restrained tears." 
"In the circumstances he felt he'd been very restrained." 
"Livy thought Caroline's greeting seemed a little restrained.
"Her black suit was restrained and expensive." 
"She knew he heard the restrained emotion by his pause." 
"This time, she sensed restrained power of a different kind." 
"He restrained himself as much as possible to keep from injuring Ully's test subject."  
"He restrained the urge to reach out to her." 
"God, acting as a good king and a true father, has given us a will which cannot be restrained, compelled or thwarted." 

According to the Oxford English Dictionary (OED), the noun restraint was first used in English around 1439–1440. The earliest evidence cited by the OED comes from the Rolls of Parliament: Henry VI. 

Examples of "restraint" used in a sentence:  
"He says Earnhardt died because he was not wearing a head and neck restraint." 
"Never place a rear facing child restraint in front of a passenger side air bag." 
"You need to exercise care and restraint when you slip the CPU into the socket." 
"The paramedics now have him turned over and he is in a neck restraint."

Restrained is one of those words that can refer to a person not being able to do something because of external factors or because of internal factors.

A person may be restrained from touching a snake because of a physical barrier between them and the snake or because of an internalized fear of snakes.

The statement "he is restrained from speaking to you" is beautifully ambiguous.  It can mean anything from he is tied up and gagged, through he is legally prevented from speaking, to he has personal reasons for not speaking.