There are lots of books with really cool old words that are no longer in standard print dictionaries. Is it all right to use them in writing and speech?
Remember that antiquated words can be revived! It is never wrong to use older or obsolete words or phrases, but it might make your speech and writing harder for others to understand. The English language is always in a state of flux. New words are popping up as you are reading this and hundreds of great old words are being ignored. Dictionaries let fledgling terms in and dismiss antiquated phrasings. Look through a few of the many great books describing lost words and you will see the richness of these great old expressions. Since language is among the most durable links we have to people of earlier times, it is great to hold on to as many of these words and phrases as are useful to you. There are many interesting and forgotten expressions which can lead to a better understanding of the people who used these terms and the times in which they lived. The English language becomes more multidimensional when curious old words are appreciated by word-gatherers. So, the answer is that you can use words and phrases that have been omitted from standard dictionaries - but carefully. The context in which you use them should make their meanings clear to the uninitiated.