The word "welkin" comes from the Old English word wolcen, meaning "cloud" or 'sky".  It entered Middle English as welken (also meaning cloud, or the heavens).  It is part of a larger Germanic family of words that have the same linguistic derivation with Dutch wolk and German Wolke, both meaning "cloud".  The word is primarily poetic and archaic today, with the common phrase "to make the welkin ring" describing a very loud sound.  

According to the Oxford English Dictionary (OED), the noun welkin first appeared in English during the Old English period, predating 1150, and is derived from Germanic languages. The OED's first documented use of the word is in the Old English period.  Other sources say the earliest recorded use of welkin is before 900 CE.  The word then evolved into Middle English welkne, and its first known use as welkin to mean the sky or heavens dates to the 12th century.  Geoffrey Chaucer wrote, "Till on the welkin shone the starres bright" in The Canterbury Tales. 

Examples of the word "welkin" used in a sentence:  
"As the sun set, the welkin was painted with hues of orange and pink, a breathtaking sight."  
"Looking up at the endless welkin, the poet imagined stars as distant islands in a vast ocean." 
"The triumphant shouts of the crowd seemed to make the very welkin ring with joy." 
"As they drove through the night, the North Star arose in the darkened welkin, guiding them home." 

Shakespeare used the word "welkin" in several of his plays: 

Just the facts:

WELKIN Definition & Meaning - Merriam-Webster