The word liqueur comes from the Latin, 'liquefacere' - to melt or dissolve, in this case referring to the a base spirit where fruits, herbs and plants are distilled as alcoholic beverages, most often sweetened.
Born in Spain in 1240, Catalan Arnold de Vila Nova was an alchemist who is noted as the man whose writings were on the first flavoured alcohols and who wrote the famous book, 'The Book of Wine'. Within it, he wrote of distilling wine into eaux de vie, and their being flavoured with spices and herbs and describes their life-giving properties!
Their use was soon to become less medicinal and more for enjoyment, spreading throughout the Mediterranean countries of Europe and beyond during the 14th Century, and became popular with the nobility of the period, thereby ensuring their popularity would 'trickle down' to the masses.
The next three centuries saw liqueurs produced by alchemist monks from various monastic orders including Benedictine and Chartreuse among many others until the true commercialisation of distilling was undertaken by the likes of the Dutch company Bols, founded in 1745, where their first production liqueur was an anisette.
Production techniques evolved to include maceration (steeping the flavours in alcohol), distillation of the macerated liquid, and percolation where the base spirit is dripped through the base flavours where it is either heated or/and is steamed through, much in the same way as we might percolate coffee. Different flavours can be taken from the fruit, spice, or herb depending on the type of distillation method chosen.
Liqueurs are now produced all over the world, often reflecting their cultural roots and the flavours they are famous for, to the more wacky and way-out shot-liqueurs we are all used to now in the modern age.